John Calvin Commentary Leviticus 16:1

"And Jehovah spake unto Moses, after the death of the two sons of Aaron, when they drew near before Jehovah, and died;" — Leviticus 16:1 (ASV)

And the Lord spoke to Moses.

A detailed description is given here of what we recently referred to briefly, so to speak, that is, the solemn atonement that was made yearly in the seventh month. For when Moses was instructing them about which sacrifices were to be offered on each of the festivals, he expressly excluded, though only in a single word, this sacrifice, when he spoke of the day of atonement itself, on which they afflicted their souls. Therefore, a clear and distinct exposition of it is now separately given.

For although their public and private sins were also expiated at other seasons of the year, and the daily sacrifices served this purpose, this more solemn rite was nevertheless intended to stir the people’s minds, so that they might more earnestly apply themselves throughout the year to the diligent seeking of pardon and remission. Then, so that they might be more earnest in propitiating God, one atonement was performed at the end of the year that would ratify all the others.

But, so that they might more diligently observe what is commanded, Moses mentions the time in which the Law was given, namely, when Nadab and Abihu were put to death by God, after they had rashly defiled the altar by their negligence.